Intraneuronal amyloid beta accumulation and oxidative damage to nucleic acids in Alzheimer disease

RNA modifications by oxidation: a novel disease mechanism?

The past decade has provided exciting insights into a novel class of central (small) RNA molecules intimately involved in gene regulation. Only a small percentage of our DNA is translated into proteins by mRNA, yet 80% or more of the DNA is transcribed into RNA, and this RNA has been found to encompass various classes of novel regulatory RNAs, including, e.g., microRNAs. It is well known that DNA is constantly oxidized and repaired by complex genome maintenance mechanisms. Analogously, RNA also undergoes significant oxidation, and there are now convincing data suggesting that oxidation, and the consequent loss of integrity of RNA, is a mechanism for disease development. Oxidized RNA is found in a large variety of diseases, and interest has been especially devoted to degenerative brain diseases such as Alzheimer disease, in which up to 50-70% of specific mRNA molecules are reported oxidized, whereas other RNA molecules show virtually no oxidation. The iron-storage disease hemochromatosis exhibits the most prominent general increase in RNA oxidation ever observed. Oxidation of RNA primarily leads to strand breaks and to oxidative base modifications. Oxidized mRNA is recognized by the ribosomes, but the oxidation results in ribosomal stalling and dysfunction, followed by decreased levels of functional protein as well as the production of truncated proteins that do not undergo proper folding and may result in protein aggregation within the cell. Ribosomal dysfunction may also signal apoptosis by p53-independent pathways. There are very few reports on interventions that reduce RNA oxidation, one interesting observation being a reduction in RNA oxidation by ingestion of raw olive oil. High urinary excretion of 8-oxo-guanosine, a biomarker for RNA oxidation, is highly predictive of death in newly diagnosed type 2 diabetics; this demonstrates the clinical relevance of RNA oxidation. Taken collectively the available data suggest that RNA oxidation is a contributing factor in several diseases such as diabetes, hemochromatosis, heart failure, and β-cell destruction. The mechanism involves free iron and hydrogen peroxide from mitochondrial dysfunction that together lead to RNA oxidation that in turn gives rise to truncated proteins that may cause aggregation. Thus RNA oxidation may well be an important novel contributing mechanism for several diseases.

The earliest stage of cognitive impairment in transition from normal aging to Alzheimer disease is marked by prominent RNA oxidation in vulnerable neurons

Although neuronal RNA oxidation is a prominent and established feature in age-associated neurodegenerative disorders such as Alzheimer disease (AD), oxidative damage to neuronal RNA in aging and in the transitional stages from normal elderly to the onset of AD has not been fully examined. In this study, we used an in situ approachto identify an oxidized RNA nucleoside 8-hydroxyguanosine (8OHG) in the cerebral cortex of 65 individuals without dementia ranging in age from 0.3 to 86 years. We also examined brain samples from 20 elderly who were evaluated for their premortem clinicaldementia rating score and postmortem brain pathologic diagnoses to investigate preclinical AD and mild cognitive impairment. Relative density measurements of 8OHG-immunoreactivity revealed a statistically significant increase in neuronal RNA oxidation during aging in the hippocampus and the temporal neocortex. In subjects with mild cognitive impairment but not preclinical AD, neurons of the temporal cortex showed a higher burden of oxidized RNA compared to age-matched controls. These results indicate that, although neuronal RNA oxidation fundamentally occurs as an age-associated phenomenon, more prominent RNA damage than in normal aging correlates with the onset of cognitive impairment in the prodromal stage of AD.

Amyloid Beta and tau proteins as therapeutic targets for Alzheimer's disease treatment: rethinking the current strategy

Alzheimer's disease (AD) is defined by the concurrence of accumulation of abnormal aggregates composed of two proteins: Amyloid beta (Aβ) and tau, and of cellular changes including neurite degeneration and loss of neurons and cognitive functions. Based on their strong association with disease, genetically and pathologically, it is not surprising that there has been a focus towards developing therapies against the aggregated structures. Unfortunately, current therapies have but mild benefit. With this in mind we will focus on the relationship of synaptic plasticity with Aβ and tau protein and their role as potential targets for the development of therapeutic drugs. Finally, we will provide perspectives in developing a multifactorial strategy for AD treatment.

Frontiers in Alzheimer's disease therapeutics

Alzheimer disease (AD) is a progressive neurodegenerative disease which begins with insidious deterioration of higher cognition and progresses to severe dementia. Clinical symptoms typically involve impairment of memory and at least one other cognitive domain. Because of the exponential increase in the incidence of AD with age, the aging population across the world has seen a congruous increase AD, emphasizing the importance of disease altering therapy. Current therapeutics on the market, including cholinesterase inhibitors and N-methyl-D-aspartate receptor antagonists, provide symptomatic relief but do not alter progression of the disease. Therefore, progress in the areas of prevention and disease modification may be of critical interest. In this review, we summarize novel AD therapeutics that are currently being explored, and also mechanisms of action of specific drugs within the context of current knowledge of AD pathologic pathways.

Inducing autophagy by rapamycin before, but not after, the formation of plaques and tangles ameliorates cognitive deficits

Previous studies have shown that inducing autophagy ameliorates early cognitive deficits associated with the build-up of soluble amyloid-β (Aβ). However, the effects of inducing autophagy on plaques and tangles are yet to be determined. While soluble Aβ and tau represent toxic species in Alzheimer's disease (AD) pathogenesis, there is well documented evidence that plaques and tangles also are detrimental to normal brain function. Thus, it is critical to assess the effects of inducing autophagy in an animal model with established plaques and tangles. Here we show that rapamycin, when given prophylactically to 2-month-old 3xTg-AD mice throughout their life, induces autophagy and significantly reduces plaques, tangles and cognitive deficits. In contrast, inducing autophagy in 15-month-old 3xTg-AD mice, which have established plaques and tangles, has no effects on AD-like pathology and cognitive deficits. In conclusion, we show that autophagy induction via rapamycin may represent a valid therapeutic strategy in AD when administered early in the disease progression.

High-resolution 3D reconstruction reveals intra-synaptic amyloid fibrils

β-Amyloid (Aβ) accumulation and aggregation are hallmarks of Alzheimer's disease (AD). High-resolution three-dimensional (HR-3D) volumetric imaging allows for better analysis of fluorescence confocal microscopy and 3D visualization of Aβ pathology in brain. Early intraneuronal Aβ pathology was studied in AD transgenic mouse brains by HR-3D volumetric imaging. To better visualize and analyze the development of Aβ pathology, thioflavin S staining and immunofluorescence using antibodies against Aβ, fibrillar Aβ, and structural and synaptic neuronal proteins were performed in the brain tissue of Tg19959, wild-type, and Tg19959-YFP mice at different ages. Images obtained by confocal microscopy were reconstructed into three-dimensional volumetric datasets. Such volumetric imaging of CA1 hippocampus of AD transgenic mice showed intraneuronal onset of Aβ42 accumulation and fibrillization within cell bodies, neurites, and synapses before plaque formation. Notably, early fibrillar Aβ was evident within individual synaptic compartments, where it was associated with abnormal morphology. In dendrites, increasing intraneuronal thioflavin S correlated with decreases in neurofilament marker SMI32. Fibrillar Aβ aggregates could be seen piercing the cell membrane. These data support that Aβ fibrillization begins within AD vulnerable neurons, leading to disruption of cytoarchitecture and degeneration of spines and neurites. Thus, HR-3D volumetric image analysis allows for better visualization of intraneuronal Aβ pathology and provides new insights into plaque formation in AD.

Protective effects of walnut extract against amyloid beta peptide-induced cell death and oxidative stress in PC12 cells

Amyloid beta-protein (Aβ) is the major component of senile plaques and cerebrovascular amyloid deposits in individuals with Alzheimer’s disease. Aβ is known to increase free radical production in neuronal cells, leading to oxidative stress and cell death. Recently, considerable attention has been focused on dietary antioxidants that are able to scavenge reactive oxygen species (ROS), thereby offering protection against oxidative stress. Walnuts are rich in components that have anti-oxidant and anti-inflammatory properties. The inhibition of in vitro fibrillization of synthetic Aβ, and solubilization of preformed fibrillar Aβ by walnut extract was previously reported. The present study was designed to investigate whether walnut extract can protect against Aβ-induced oxidative damage and cytotoxicity. The effect of walnut extract on Aβ-induced cellular damage, ROS generation and apoptosis in PC12 pheochromocytoma cells was studied. Walnut extract reduced Aβ-mediated cell death assessed by MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) reduction, and release of lactate dehydrogenase (membrane damage), DNA damage (apoptosis) and generation of ROS in a concentration-dependent manner. These results suggest that walnut extract can counteract Aβ-induced oxidative stress and associated cell death.

Aβ oligomer-induced synapse degeneration in Alzheimer's disease

Aβ oligomers cause a collection of molecular events associated with memory loss in Alzheimer's disease, centering on disrupting the maintenance of synapse structure and function. In this brief review of the synaptotoxic effects of Aβ oligomers, we focus on the neuronal properties governing oligomer targeting and toxicity-especially with respect to binding sites and mechanisms of binding. We also discuss ways in which mechanistic insights from other diseases offer clues in the pursuit of the molecular basis of Alzheimer's disease.

Mitochondrial dysfunction and Alzheimer's disease

To date, one of the most discussed hypotheses for Alzheimer's disease (AD) etiology implicates mitochondrial dysfunction and oxidative stress as one of the primary events in the course of AD. In this review we focus on the role of mitochondria and mitochondrial DNA (mtDNA) variation in AD and discuss the rationale for the involvement of mitochondrial abnormalities in AD pathology. We summarize the current data regarding the proteins involved in mitochondrial function and pathology observed in AD, and discuss the role of somatic mutations and mitochondrial haplogroups in AD development.

A mitocentric view of Alzheimer's disease suggests multi-faceted treatments

Alzheimer's disease (AD) is defined by senile plaques made of amyloid-beta peptide (Abeta), neurofibrillary tangles made of hyperphosphorylated tau proteins, and memory deficits. Thus, the events initiating the cascade leading to these end points may be more effective therapeutic targets than treating each facet individually. In the small percentage of cases of AD that are genetic (or animal models that reflect this form of AD), the factor initiating AD is clear (e.g., genetic mutations lead to high Abeta1-42 or hyperphosphorylated tau proteins). In the vast majority of AD cases, the cause is unknown. Substantial evidence now suggests that abnormalities in glucose metabolism/mitochondrial function/oxidative stress (GMO) are an invariant feature of AD and occur at an early stage of the disease process in both genetic and non-genetic forms of AD. Indeed, decreases in brain glucose utilization are diagnostic for AD. Changes in calcium homeostasis also precede clinical manifestations of AD. Abnormal GMO can lead to plaques, tangles, and the calcium abnormalities that accompany AD. Abnormalities in GMO diminish the ability of the brain to adapt. Therapies targeting mitochondria may ameliorate abnormalities in plaques, tangles, calcium homeostasis, and cognition that comprise AD.

X-ray induced cataract is preceded by LEC loss, and coincident with accumulation of cortical DNA, and ROS; similarities with age-related cataracts

PURPOSE

To compare age-related cataractous (ARC) changes in unirradiated mice lenses to those induced by head-only X-irradiation of 3 month-old mice.

METHODS

lens epithelial cells (LECs) as well as partially degraded cortical DNA were visualized in fixed sections using 4',6-diamidino-2-phenylindole (DAPI) staining, and in fresh lenses using the vital stain Hoechst 33342. reactive oxygen species (ROS) activity was also visualized directly in fresh lenses using the vital dye Dihydrorhodamine (DHR). In fixed lenses an antibody specific for 8-OH Guanosine (8-OH-G) lesions was used to visualize DNA oxidative adducts from ROS damage. Alpha smooth muscle actin was visualized using specific antibodies to determine if myofibroblasts were present. Fluorescence was quantified using Laser Scanning Confocal Microscopy (LSCM). The degree of lens opacity and cataract formation was determined by slit lamp, or from digitalized images of light reflections taken with a low magnification light microscope.

RESULTS

Using DNA- and ROS-specific vital fluorescent dyes, and laser scanning confocal microscopy we have previously described 4 changes in the aging rodent lenses: 1) a significantly decreased density of surface LECs in lenses from old compared to younger mice and rats; 2) a very large increase in retained cortical nuclei and DNA fragments in the secondary lens fibers of old rodent lenses; 3) increased cortical ROS in old rodent lenses; 4) increased cataract concomitantly with the cortical DNA and ROS increases. In the current study we report that these same 4 changes also occur in an accelerated fashion in mice given head-only X-irradiation at 3 months of age. In addition to vital staining of fresh lenses, we also examined sections from fixed eyes stained with DAPI or hematoxylin and eosin (H&E) and found the same loss of surface LECs and accumulation of undigested nuclei and debris in secondary lens fibers occur with age or following X-irradiation. In addition sections from fixed-eyes were examined for ROS damage to DNA with antibodies specific for 8-OH-G lesions. The frequency of 8-OH-G lesions increased dramatically in lenses from old unirradiated mice over 24 months of age, and similarly in X-irradiated lenses by 9-11 months post irradiation. The accumulation of cortical nuclei was not the result of conversion or invasion by myofibroblasts as tested by antibodies to a marker for such cells, alpha smooth muscle actin.

CONCLUSIONS

X-irradiation damage induces a large decrease in surface LECs over a period of 3-11 months post X-irradiation of young mice. These changes are similar in extent to those seen in 24-29 months-old control mouse lenses with age-related cataracts. In 24+ month-old unirradiated mice the secondary lens fibers are not able to degrade nuclei or nuclear DNA efficiently and accumulate large numbers of cortical nuclei and nuclear fragments as well as ROS and 8-OHG lesions. X-irradiated lenses develop the same abnormalities in a more accelerated fashion. The extensive loss of LECS and accumulation of undegraded nuclei, ROS, and ROS damage may play a causal role in cataract generation in both unirradiated old mice and in previously irradiated young adult mice.